Fusobacterium necrophorum, classified into subsp. necrophorum (Fnn) and subsp. funduliforme (Fnf), is frequently associated with necrotic infections of animals and humans. The outer membrane proteins (OMP) of many Gram negative bacteria play an important role in bacterial adhesion and establishment of infection. The OMP profile of F. necrophorum has not been well characterized. We analyzed OMP of bovine strains of Fnn and Fnf and human strains of F. necrophorum. Electrophoretic separations of extracted OMP of Fnn and Fnf strains of cattle showed a total of 19 and 20 protein bands, respectively. The most prominent protein band was 40 kDa in Fnn and 37.5 kDa in Fnf. The four human clinical strains examined had more heterogeneous banding patterns and had different profiles than those of bovine Fnf strains. A total of 11 protein bands in Fnn and 13 protein bands in Fnf were recognized by sera from cattle with liver abscesses. The intensities of many of the bands in Fnn were higher than that of Fnf. We conclude that the two subspecies of F. necrophorum differ in their OMP profiles and the difference may account for differences in their virulence and involvement in the pathogenesis of necrotic infections.